Sheet conveyor, image forming apparatus, and sheet conveying method

ABSTRACT

According to one embodiment, a sheet conveyor includes a first roller  62  having a first cam  71 , rotating against the first roller, whose cam diameter simply increases and a second roller  63  having a second cam  72 , rotating against the second roller, whose cam diameter simply increases. The first cam contacts the second cam at their cam surfaces of increased cam diameters when the sheet is conveyed by the first and second rollers. When the rear end of the sheet goes out of the first and second rollers, a delay time is provided by the first and second cams until the first and second rollers directly contact one the other because the first and second cams rotate by the same distance from the contacting cam surfaces of increased cam diameter.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior U.S. Patent Application No. 61/266,590, filed on Dec. 4, 2009, the entire contents of which are incorporated herein by reference.

This application is also based upon and claims the benefit of priority from Japanese Patent Application No. 2010-196271, filed on Sep. 2, 2010, the entire contents of which are incorporated herein by reference.

FIELD

Exemplary embodiments described herein relate to a sheet conveyor, an image forming apparatus having the sheet conveyor and a sheet conveying method

BACKGROUND

An image forming apparatus such as a color copy machine or a printer includes a plurality of image forming units along an intermediate transfer medium in a belt shape or sheet shape. This kind of image forming apparatus superimpose a plurality of color toner images formed by the respective image forming units on the intermediate transfer medium and transfer them onto a sheet in a batch thereby obtaining a color image.

Such an image forming apparatus has a pair of rollers called an aligning roller pair located just before the image transfer position where a toner image on the intermediate transfer medium is transferred. Sheet being conveyed to the image transfer position is stopped temporarily before the image transfer position by the aligning roller pair that is not driven. Afterward the aligning roller pair is driven and thus conveys sheet to the image transfer position. In this way, a method that the sheet conveyed is stopped once before the image transfer position and a sheet position to the toner image on the medium is adjusted so that the toner image on the medium is transferred onto the sheet at an appropriate position is used widely.

When a rear end of the sheet being conveyed by the aligning roller pair leaves the aligning roller pair, the speed of the sheet conveyed to the transfer position suddenly changes because the rear end of the sheet is pushed out with a force generated when the roller pair forcibly contacts each other by a pressure applied thereto. A displacement in position of the toner image transferred to the sheet occurs by the change of such conveying speed.

To avoid a rapid increase in the conveying speed of the sheet, there is a system in which one of the aligning roller pair has a cam. The cam freely rotates for the one of the roller pair and is given a force in the opposite direction to the rotational direction of the one of the roller pair while the cam contacts the other roller.

In the system described above, the surface of the cam is apt to be abraded while the other roller rotates because of the stopping the cam. Therefore an embodiment of the present invention aims to provide a sheet conveyor which decreases the burden on the cam being utilized to prevent the conveying speed of the sheet from increasing rapidly when the rear end of the sheet conveyed is pressed out from the aligning roller pair.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing an example of the image forming apparatus to which an embodiment of the present invention is applied;

FIG. 2 is a perspective view showing the constitution of an aligning roller pair according to the first embodiment;

FIG. 3 is a schematic view illustrating operation if the pressurizing delay mechanism according to the first embodiment;

FIG. 4 is a schematic view illustrating operation of the pressurizing delay mechanism following FIG. 3;

FIG. 5 is a schematic view illustrating further operation of the pressurizing delay mechanism following FIG. 4;

FIG. 6 is a schematic view illustrating further operation of the pressurizing delay mechanism following FIG. 5;

FIG. 7 is a schematic view illustrating further operation of the pressurizing delay mechanism following FIG. 6;

FIG. 8 is a schematic view illustrating the pressurizing delay mechanism according to the second embodiment;

FIG. 9 is a perspective view illustrating a part of the pressurizing delay mechanism according to the third embodiment; and

FIG. 10 is an enlarged perspective view of the cam shown in FIG. 9;

DETAILED DESCRIPTION

In general, according to one embodiment, there is provided sheet conveyor includes a first roller which rotates in a first rotational direction; a second roller which contacts with the first roller to rotate in a second rotational direction opposite to the first rotational direction, the second roller and the first roller conveying a sheet by holding the sheet therebetween; a first cam configured to move with the first roller and to rotate against the first roller, the first cam having a cam surface of a first diameter when taking a first cam posture in a state that the sheet is not present between the first and second rollers, and having a cam surface of a second diameter greater than the first diameter when taking a second cam posture in a state that the sheet is present between the first and second rollers; a second cam configured to move with the first roller and to rotate against the second roller, the second cam having a cam surface of the third diameter when taking a third cam posture in a state that the sheet is not present between the first and second rollers, the cam surface of the fourth diameter of the second cam when taking a fourth cam posture in a state that the sheet is present between the first and second rollers, the cam surface of the third diameter of the second cam contacting the cam surface of the first diameter of the first cam when the first cam takes the first cam posture and the second cam takes the third cam posture, the cam surface of the fourth diameter of the second cam further contacting the cam surface of the second diameter if the first cam when the first cam takes the second cam posture and the second cam takes the fourth cam posture; and an urging member configured to apply a force to the first cam to rotate the first cam against the first roller from the first cam posture to the second cam posture and also apply a force to the second cam to rotate the second cam against the second roller from the third cam posture to the fourth cam posture.

Hereinafter, embodiments of the present invention will be disclosed with reference to the accompanying drawings.

First Embodiment

A first embodiment of the present invention will be explained with reference to FIGS. 1 to 7.

FIG. 1 schematically shows an example of the image forming apparatus to which embodiments of the present invention are applied. As shown in FIG. 1, the image forming apparatus 1 has a scanner unit 2 and an image forming unit 3. The scanner unit 2 scans an original document to get image information thereof, and produces an image signal based on the image information. The image forming unit 3 forms a developer image (hereinafter, referred to as a toner image) on a sheet P as an output image based on the image signal

The image forming apparatus 1 also has a plurality of sheet cassettes 4 below the image forming unit 3 for storing a plurality of sheets and feeding sheet P one by one from each sheet cassette 4. A selected one of the sheet cassettes 4 supplies sheet P of a desired size in compliance with the timing at which the image forming unit 3 forms output images.

Between the sheet cassettes 4 and image forming unit 3, there is a conveying path 5 for guiding the sheets P from the sheet cassettes 4 to the image forming unit 3. The conveying path 5 further guides the sheet P to a fixing device 6 for fixing the toner image transferred to the sheet P on the sheet P via an image transfer position 5A where a toner image formed by the image forming unit 3 is transferred to the sheet P.

The image forming unit 3 has, for example, an intermediate transfer belt 11 of an insulating film formed in an endless belt shape. The intermediate transfer belt 11 may use a sheet-shaped thin metal whose surface is protected with resin. The intermediate transfer belt 11 is given a predetermined tension by a driving roller 12, a first tension roller 13, and a second tension roller 14. Any one of the positions of the intermediate transfer belt 11 which is in parallel with the axis of the driving roller 12 is moved in the direction of an arrow A by rotation of the driving roller 12. In other words, the intermediate transfer belt 11 is circulated in one direction at a speed corresponding to the moving speed of the outer peripheral surface of the driving roller 12.

In a section in which surface of the intermediate transfer belt 11 is moved substantially in a plane in the state that a predetermined tension is given by rollers 12, 13 and 14, first to fourth image forming units 21, 22, 23 and 24 are arranged at a predetermined interval. Further, in the example shown in FIG. 1, in the section in which the surface of the intermediate transfer belt 11 is moved substantially in a plane between the driving roller 12 and the first tension roller 13, the first image forming unit 21 is positioned at a side of the driving roller 12 and the fourth image forming unit 24 at a side of the first tension roller 13.

The first to fourth image forming units 21 to 24 each has at least a developing device which correspondingly stores toner of one of colors of C (cyan), M (magenta), Y (yellow) and BK (black) and a photoconductor, e.g., photosensitive drum, for holding an electrostatic image to be developed by the developing device. On the respective photoconductors of the image forming unit, electrostatic images to be developed with respective color toners in the developing devices are respectively formed by corresponding image light from an exposure unit 31 and visible color toner images are formed by the corresponding developing devices.

Transferring rollers 41 to 44 for transferring the toner images formed on the respective photoconductors of the image forming units 21 to 24 to the intermediate transfer belt 11 are arranged at the rear side of the intermediate transfer belt 11. In the respective image forming units 21 to 24, the electrostatic images are formed at predetermined timing and are developed by the developing devices so that the toner images transferred sequentially are superimposed on the transfer belt to be a full color image.

The image transfer position 5A is formed on the conveying path 5 by the intermediate transfer belt 11 and transfer roller 51 forcibly contacting the intermediate transfer belt 11. The color image on the intermediate transfer belt 11 is transferred to the sheet P conveyed to the image transfer position 5A. The transfer roller 51 is, however, at a standby position far from the intermediate transfer belt 11 by a roller retreating mechanism (not shown) when the transfer operation is not required.

An aligning roller pair 61 is provided at a predetermined position on the conveying path 5 between the sheet cassette 4 and the image transfer position 5A. The aligning roller pair 61 stops temporarily the sheet P which is conveyed from the sheet cassette 4 toward the image transfer position 5A. One of the aligning roller pair 61 (driving roller) rotates in a predetermined direction (paper conveying direction) and the other roller (driven roller) of the aligning roller pair 61 is pressed to the driving roller with a predetermined pressure to be rotated.

The sheet P conveyed from the sheet cassette 4 toward the image transfer position 5A is stopped once by the aligning roller pair 61 that is not driven, and thus the inclination of the sheet P in the conveying direction which may occur during conveyance of the sheet P from the sheet cassette 4 along the conveying path 5 is corrected.

By controlling the timing of the rotation of the aligning roller pair 61, the timing of arrival of the sheet P at the image transfer position 5A to the color toner image on the intermediate transfer belt 11 that approaches the image transfer position 5A can be adjusted. By this operation, the position of the toner image to the sheet P is set appropriately. In other words, the position of the color toner image transferred on the sheet P can be set desirously.

The sheet P conveyed at a regular speed by the aligning roller pair 61 is conveyed toward the fixing device 6 while the color toner image is transferred thereto at the image transfer position 5A. In this process, at the moment the rear end of the sheet P comes off the aligning roller pair 61, the rear end of the sheet P is given pressure in the conveying direction by the roller pair 61 which comes to forcibly contact each other because the thickness of the rear end of the sheet P between the roller pair 61 suddenly goes to zero.

At this time, the conveying speed of the sheet P is increased temporarily because the rear end of the sheet P is pushed out in the conveying direction by the roller pair 61. Accordingly, the sheet P is forwarded rapidly to the fixing device 6 irrespective of the color toner image being transferred thereto at the image transfer position 5A. This causes a position displacement namely a transfer displacement, of the color toner image transferred onto the sheets P at the image transfer position 5A. Further, when the image output is continued two times or more, at the time of occurrence of the transfer displacement, a position displacement of the image light may also occur in the image forming unit in which the exposure process is being executed by the image light.

In this embodiment, cams are respectively provided to the aligning roller pair 61 to prevent the conveying speed of the sheet P from changing rapidly when the rear end of the sheet P is pressed out from the aligning roller pair 61. A detail of the cams will be described below.

FIG. 2 is a perspective view showing the constitution of the aligning roller pair according to the first embodiment.

As shown in FIG. 2, the aligning roller pair 61 is composed of a first roller 62 and a second roller 63 which closely contact each other with a predetermined pressure therebetween. And the first roller 62 rotates in the direction indicated by an arrow B and the second roller 63 rotates in the direction indicated by an arrow C opposite to the direction of the arrow B. The first roller 62 has a compression spring 65 as an urging member to give a predetermined pressure to the rotary shaft 62 a. And the second roller 63 also has a compression spring 66 as an urging member to give a predetermined pressure to the rotary shaft 63 a so that each contact portion of the aligning roller pair 61 is pressed one another. The above-described compression spring may be provided to one of the rotary shafts 62 a and 63 a of the aligning roller pair 61. And the urging member is not limited to a compression spring.

A first cam 71 is coaxially arranged at the predetermined position of the rotational shaft of the first roller 62. The first cam 71 moves with the first roller 62 and rotates against the first roller 62. A second cam 72 is also coaxially arranged at the predetermined position of the rotational shaft of the second roller 63. The second cam 72 moves with the second roller 63 and rotates against the second roller 63. The first cam 71 and second cam 72 are formed nearly in a fan-shape, for example. Further, they have a contact surface (cam surface) over a predetermined angle and rotate by the same distance in opposite directions respectively while they mutually contact each other with the contact surfaces.

In the first embodiment, the first cam 71 and the second cam 72 are coaxially at the rotational shafts of the first and second rollers 62 and 63, respectively. However, it is not limited to arrange the first cam 71 (the second cam 72) and the first roller 62 (the second roller 63), coaxially and thus, the first and second cams 71 and 72 may respectively rotate against the first and second rollers 62 and 63.

Structures of the first cam 71 and the second cam 72 is described below. In this embodiment, the second cam 72 is formed and arranged as a mirror image of the first cam 71. The compression spring 73 (hereinafter referred to as a first spring) gives the first cam 71 a force in the direction indicated by an arrow D1 opposite to the rotational direction of the first roller 62. Also, the compression spring 74 (hereinafter referred to as a second spring) gives the second cam 72 a force in the direction indicated by an arrow E1 opposite to the rotational direction of the second roller 63. And the second cam 72 rotates as it always contacts with the first cam 71

Further, the first cam 71 has different diameters at a side of an arrow D1 and a side of an arrow D2 (direction opposite to the arrow D1). When the sheet P is not present between the first roller 62 and the second roller 63, the first cam 71 takes a first posture. The first cam 71 contacts the second cam 72 at the first position (cam surface) where the first cam 71 taking the first cam posture has the first diameter a. When the sheet P is conveyed between the first roller 62 and the second roller 63, the first cam 71 rotates in the direction of an arrow D1 from the first cam posture to a second cam posture. The first cam 71 contacts the second cam 72 at the second position (cam surface) where the first cam 71 taking the second cam posture has the second diameter b bigger than the first diameter a.

Further, the second cam 72 has different diameters at a side of an arrow E1 and a side of an arrow E2 (opposite direction to the arrow E1).

The fourth diameter d at a fourth position on the side of the arrow E1 is bigger than the third diameter c at a third position of the arrow E2. Further, the cam surface is formed such that diameter of the second cam 72 simply increases from the third position to the fourth position.

When sheet P is not present between the first roller 62 and the second roller 63, the second cam 72 takes a third cam posture. The second cam 72 contacts the first cam 71 at the third position (cam surface) where the second cam 72 taking the third cam posture has the third diameter c. When the sheet P is conveyed between the first roller 62 and the second roller 63, the second cam 72 rotates in the direction of an arrow E1 and comes to take a fourth cam posture. When the second cam 72 comes to take the fourth posture, the second cam 72 contacts the first cam 71 at the fourth position (cam surface) where the second cam 72 has the fourth diameter d.

The above-described cam configurations prevent the conveying speed of the sheet P from changing rapidly when the rear end of the sheet P is pressed out from the aligning roller pair 61. A detailed operation is described below.

In this embodiment, the first cam 71 is the mirror image to the second cam 72. Therefore the first diameter of the first cam at the first position is equal to the third diameter c of the second cam 72 at the third position. And the second diameter b of the first cam 71 at the second position is equal to the fourth diameter d of the second cam 72 at the fourth position. However, the first cam 71 is not limited to the mirror image to the second cam 72. The diameter of the first cam 71 may be different from the diameter of the second cam 72.

Further, the above-described first cam 71 and second cam 72 is arranged to the opposite side of the each shaft of the first roller 62 and the second roller 63. Therefore the aligning roller pair 61 has four cams.

FIGS. 3 to 7 are schematic views explaining the movement of the first cam 71 and the second cam 72 from a state in which the front end of the sheet P comes into the aligning roller pair 61 to state in which the rear end of the sheet P is pressed out from the aligning roller pair 61.

FIG. 3 illustrates the situation that sheet P rushes into the aligning roller pair 61. The sheet P is not present between the first roller 62 and the second roller 63 until the sheet P rushed into the aligning roller pair 61. Therefore the first cam 71 takes the first cam posture and the second cam 72 takes the second cam posture.

When the sheet P rushes into the aligning roller pair 61, the aligning roller pair 61 is applied with a force in the direction of the arrow L because of the thickness of the sheet P that breaks the contact between rollers 61. And a gap equal to the thickness of the sheet P is produced between the first roller 62 and the second roller 63. In response to the gap produced, the first cam 71 moves with the first roller 62 and thus rotates in the direction of the arrow D1 by the first spring 73 and the second cam 72 also moves with the second roller 63 and thus rotates in the direction of the arrow E1 by the second spring 74, as shown in FIG. 4. As described above, the diameter of the first cam 71 increases in the direction opposite to the arrow D1, and the diameter of the second cam 72 increases in the direction opposite to the arrow E1. Therefore, when the gap of the aligning roller pair 61 is created, each cam rotates by the same distance along each cam surface in the direction of arrow D1 and arrow E1 respectively to contact each other at the position where each cam has bigger diameter (b, d).

As shown in FIG. 5, the first cam 71 and the second cam 72 stop at the position at which they rotate by a distance depending on the thickness of the sheet P when the sheet P is conveyed between the aligning roller pair 61. And that is, the first cam 71 stops while taking the second cam posture and the second cam 72 also stops while taking the fourth cam posture. At the moment, the force applied to the first cam 71 and the force applied to the second cam 72 are well-balanced in the direction indicated by an arrow M.

FIG. 6 shows the situation that the rear end of the sheet P is being pushed out of the aligning roller pair 61. At this moment, the force that forcibly separates the aligning roller pair 61 one another is suddenly gone. However, forces are respectively applied by the compression springs 65 and 66 to cam surfaces of the first and second cams 71 and 72 contacting each other in the direction indicated by the arrow M. FIG. 7 shows the situation just after the sheet P pressed out from the aligning roller pair 61. As previously explained, the diameters of the cams 71 and 72 continuously change along the cam surface. And cams 71 and 72 respectively rotate by the same distance along the cam surfaces in the direction of the arrow D2 and E2 so as to come into contact at the position where they have smaller diameter. With rotation of the first cam and the second cam, rollers 62 and 63 of the aligning roller pair 61 that have a gap corresponding to the thickness of the sheet P come close and then contact one another.

After rollers 62 and 63 contact one the other, the rotation of the cams 71 and 72 stops. Then the first cam 71 returns to the first cam posture and the second cam 72 returns to the third cam posture. When the rear end of the sheet P is pressed out from the aligning roller pair 61, the rollers 62 and 63 of the aligning roller pair 61 contact one the other with a delay time. Above movement is described in more detail. The first spring 73 gives the first cam 71 a predetermined force in the direction of the arrow D1. The second spring 74 gives the second cam 72 a predetermined force in the direction of the arrow E1.

After the sheet P goes out completely from the aligning roller pair 61, the first cam 71 returns to a prescribed cam posture at a position where the sheet P is not present between the aligning roller pair 61 over a time which is defined based on the difference between a force in the direction indicated by the above-described arrow D2 and a force by the first spring 73 in the direction indicated by the arrow D1 opposite to the arrow D2. Similarly, the second cam 72 returns to a prescribed cam posture at a position where the sheet P is not present between the aligning roller pair 61 over a time which is defined based on the difference between a force in the direction indicated by the arrow E2 and a force by the second spring 74 in the direction indicated by the arrow E1 opposite to the arrow E2.

Therefore, the rollers of the aligning roller pair 61 do not immediately contact one another just after the sheet P is pressed out from the rollers. And the rollers closely contact after a fixed period of time elapses.

As described above, a cam whose diameter changes along the cam surface is provided to the first roller 62 and the second roller 63 of the aligning roller pair 61. Therefore, it is prevented that the gap which is produced with the thickness of the sheet P between the roller pair 61 suddenly goes to zero just after the rear end of the sheet P passes the first and second rollers 62 and 63. In addition, it is also prevented that the cams 71 and 72 are worn out, because the first cam 71 and second cam 72 rotate by the same distance along the cam surfaces without an excessive friction.

Further, the diameters of the cams and the repelling forces of the compression springs are set optionally on the basis of various parameters such as the number of image output sheets per a unit time, diameters of the respective rollers of the aligning roller pair, a pressure given to the aligning roller pair, etc.

In the above-mentioned embodiment, the aligning rollers are illustrated as an example. However, the first cam 71 and the second cam 72 can be applied to an aligning roller pair 7A in a sheet reversing unit 7 of the color copy machine 1 or an automatic document feeder (ADF) integrally installed in the scanner unit 2.

Second Embodiment

Next, the second embodiment will be explained by referring to FIG. 8.

FIG. 8 is a cross-sectional view illustrating a pressurizing delay mechanism according to the second embodiment

In this embodiment, urging members of the first cam and the second cam are arranged at the down stream side of the aligning roller pair in the conveying direction of the sheet P.

The diameters of the first cam 71 and the second cam 72 are described below. The first cam 71 has different diameters at a side of the arrow D1 and a side of the arrow D2, respectively. Further, the second cam 72 also has different diameters at a side of the arrow E1 and a side of the arrow E2, respectively.

When the sheet P is not present between the first roller 62 and the second roller 63, the first cam takes a first cam posture. The first cam 71 contacts the second cam 72 at the first position (cam surface) at which the first cam 71 taking the first cam posture has the first diameter a. When the sheet P is conveyed between the first roller 62 and the second roller 63, the first cam rotates in the direction of the arrow D2 to take a second cam posture. When the first cam 71 takes the second cam posture, the first cam 71 contacts the second cam 72 at the second position (cam surface) at which the first cam 71 taking the second cam posture has the second diameter b greater than the first diameter a.

The second cam 72 has the different diameters (c, d) at the side of the arrow E1 and the side of the arrow E2, respectively. When the sheet P is not present between the first roller 62 and the second roller 63, the second cam 72 takes a third cam posture. And the second cam 72 contacts the first cam 71 at the third position where the second cam 72 taking the third ca posture has the third diameter c. When the sheet P is conveyed between the first roller 62 and the second roller 63, the second cam 72 rotates from the third cam posture to a fourth cam posture in the direction of the arrow E2. When the second cam 72 comes to take the fourth cam posture, the second cam 72 contacts the first cam 71 at the fourth position where the second cam 72 taking the fourth cam posture has the fourth diameter d greater than the third diameter c.

As described above, in the second embodiment, the diameter of the first cam 71 simply increases in the direction of the arrow D2. Also, the diameter of the second cam 72 simply increases in the direction of the arrow E2. In this embodiment, the diameters of the cams 71 and 72 respectively increase in directions opposite to those of the first embodiment. And when the rear end of the sheet P is pressed out from the aligning roller pair 61, the rollers 62 and 63 of the aligning roller pair 61 contact each other with a delay time because of the above-described operation of the first and second cams 71 and 72.

As described above, in the second embodiment also that urging members of the first cam and the second cam are arranged at the down stream side of the aligning roller pair in the conveying direction of the sheet P, the cams prevent the aligning roller pair from suddenly returning to their original position which the aligning roller pair closely contact each other. Further, it is also prevented that the cams 71 and 72 are worn out, because the first cam 71 and second cam 72 rotate by the same distance along the cam surfaces without any excessive friction.

Third Embodiment

Next, the third embodiment will be explained by referring to FIGS. 9 and 10.

FIG. 9 is a schematic view illustrating the pressurizing delay mechanism according to the third embodiment. FIG. 10 is an enlarged perspective view of the cam shown in FIG. 9.

In this embodiment, the first cam 71 and the second cam 72 each has a gear 80 a formed on a part of the cam surface along its rotational direction. As shown in FIG. 10, the cam surface of each cam 71, 72 is divided into two parts along the rotational direction and the gears 80 a is formed only on one of the two parts of the cam. Therefore, gears 80 a of the first and second cams 71 and 72 are meshed with one another and other parts 80 b of the first and second cams 71 and 72 on which gears 80 a are not formed contact each other to be rotated smoothly when the first and second cams 71 and 72 rotate one another, as being similar to the first embodiment.

In this case, an urging member that gives a force to the cams 71 and 72 in opposite direction to the rotational direction of the aligning roller pair 61 may be arranged to either the first cam 71 or the second cam 72. In FIG. 9, the first cam 71 has the compression spring 73 as an urging member. When one of the cams is given a force, the other cam that is not provided with the compression spring 73 is also given the force through the gears of the first cam 71 and the second cam 72 being meshed with one another.

The movement of the first cam 71 and the second cam 72 after the sheet P rushes into the aligning roller pair 61 until it is pressed out is the same as that of the first embodiment. However, unlike the first embodiment, the cams rotate while gears are meshed with each other in this embodiment. As being similar to the first embodiment, no friction between the first cam 71 and the second cam 72 occur because the first cam 71 and the second cam 72 rotate by the same distance.

In the present embodiment also, cams each having the above-described construction are provided to the both sides of each roller of the aligning roller pair 61 respectively like the first embodiment.

According to the above-described embodiment, it is not necessary to provide the urging member to respective cams because gears of the first cam 71 and the second cam 72 are meshed with one another and a force applied to one of the cams is transmitted to the other cam through gears being meshed. Therefore the number of urging members may be reduced.

Further, in this embodiment, the gear is arranged on one of the two parts of the each cam surface that are equally divided along the rotational direction of the cam. It may also be possible to operate the cams to avoid a rapid change in speed of the sheet P when sheet P is pushed out from the aligning roller pair 61 if the gears are formed on the entire cam surfaces of the cams, respectively. However, in that case, it is difficult to move the cams smoothly in comparison with the first embodiment in which the cams are smoothly moved because of no gears on the surfaces of the cams. Therefore according to this embodiment, the gear is formed on a part of the cam surface of each cam to move the cams smoothly in comparison with a case in which the gear is formed on the entire cam surface of each cam.

While certain embodiments have been described, those embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and apparatuses described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and apparatuses described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

1. A sheet conveyor comprising: a first roller which rotates in a first rotational direction; a second roller which contacts with the first roller to rotate in a second rotational direction opposite to the first rotational direction, the second roller and the first roller conveying a sheet by holding the sheet therebetween; a first cam configured to move with the first roller and to rotate against the first roller, the first cam having a cam surface of a first diameter when taking a first cam posture in a state that the sheet is not present between the first and second rollers, and having a cam surface of a second diameter greater than the first diameter when taking a second cam posture in a state that the sheet is present between the first and second rollers; a second cam configured to move with the first roller and to rotate against the second roller, the second cam having a cam surface of the third diameter when taking a third cam posture in a state that the sheet is not present between the first and second rollers, the cam surface of the fourth diameter of the second cam when taking a fourth cam posture in a state that the sheet is present between the first and second rollers, the cam surface of the third diameter of the second cam contacting the cam surface of the first diameter of the first cam when the first cam takes the first cam posture and the second cam takes the third cam posture, the cam surface of the fourth diameter of the second cam further contacting the cam surface of the second diameter if the first cam when the first cam takes the second cam posture and the second cam takes the fourth cam posture; and an urging member configured to apply a force to the first cam to rotate the first cam against the first roller from the first cam posture to the second cam posture and also apply a force to the second cam to rotate the second cam against the second roller from the third cam posture to the fourth cam posture.
 2. The sheet conveyor according to claim 1, further comprising: a second urging member that applies a pressure to the first cam and the second cam to forcibly contact one the other.
 3. The sheet conveyor according to claim 1, wherein the first cam has a first gear and the second cam has a second gear meshed with the first gear.
 4. The sheet conveyor according to claim 3, wherein the first cam rotates against the first roller and the second cam rotates against the second roller simultaneously as the first gear and the second gear are meshed with one the other.
 5. The sheet conveyor according to claim 1, wherein the first cam has a surface distance between the cam surface of the first diameter and the cam surface of the second diameter and the second cam has a surface distance equal to that of the first cam between the cam surface of the third diameter and the cam surface of the fourth diameter.
 6. The sheet conveyor according to claim 1, wherein the diameter of the first cam simply increases from the cam surface of the first diameter to the cam surface of the second diameter, the diameter of the second cam also simply increases from the cam surface of the third diameter to the cam surface of the fourth diameter.
 7. The sheet conveyor according to claim 1, wherein the urging member includes a first spring which applies the force to the first cam and a second spring which applies the force to the second cam.
 8. The sheet conveyor according to claim 1, wherein the first roller and the second roller contact one the other with a delay time after the rear end of the sheet is pressed out from the first roller and the second roller, the delay time being a time interval from the state that the cam surface of the second diameter of the first cam taking the second cam posture contacts the cam surface of the fourth diameter of the second cam taking the fourth cam posture to the state that the cam surface of the first diameter of the first cam taking the first cam posture contacts the cam surface of the third diameter of the second cam taking the third cam posture.
 9. An image forming apparatus comprising: a sheet feeding section configured to supply a sheet; a sheet conveying section, configured to convey the sheet from the sheet feeding section to an image transfer position, which includes a first roller which rotates in a first rotational direction, a second roller which contacts with the first roller to rotate in a second rotational direction opposite to the first rotational direction, the second roller and the first roller conveying a sheet by holding the sheet therebetween, a first cam configured to move with the first roller and to rotate against the first roller, the first cam having a cam surface of a first diameter when taking a first cam posture in a state that the sheet is not present between the first and second rollers, and having a cam surface of a second diameter greater than the first diameter when taking a second cam posture in a state that the sheet is present between the first and second rollers, a second cam configured to move with the first roller and to rotate against the second roller, the second cam having a cam surface of the third diameter when taking a third cam posture in a state that the sheet is not present between the first and second rollers, the cam surface of the fourth diameter of the second cam when taking a fourth cam posture in a state that the sheet is present between the first and second rollers, the cam surface of the third diameter of the second cam contacting the cam surface of the first diameter of the first cam when the first cam takes the first cam posture and the second cam takes the third cam posture, the cam surface of the fourth diameter of the second cam further contacting the cam surface of the second diameter if the first cam when the first cam takes the second cam posture and the second cam takes the fourth cam posture, and an urging member configured to apply a force to the first cam to rotate the first cam against the first roller from the first cam posture to the second cam posture and also apply a force to the second cam to rotate the second cam against the second roller from the third cam posture to the fourth cam posture; and an image forming section which forms an image and transfers the image to the sheet conveyed by the sheet conveying section at the image transfer position.
 10. The image forming apparatus according to claim 9, further comprising: a second urging member that applies a pressure to the first cam and the second cam to forcibly contact one the other.
 11. The image forming apparatus according to claim 9, wherein the first cam has a first gear and the second cam has a second gear meshed with the first gear.
 12. The image forming apparatus according to claim 11, wherein the first cam rotates against the first roller and the second cam rotates against the second roller simultaneously as the first gear and the second gear are meshed with one the other.
 13. The image forming apparatus according to claim 9, wherein the first cam has a surface distance between the cam surface of the first diameter and the cam surface of the second diameter and the second cam has a surface distance equal to that of the first cam between the cam surface of the third diameter and the cam surface of the fourth diameter.
 14. The image forming apparatus according to claim 9, wherein the diameter of the first cam simply increases from the cam surface of the first diameter to the cam surface of the second diameter, and the diameter of the second cam also simply increases from the cam surface of the third diameter to the cam surface of the fourth diameter.
 15. The image forming apparatus according to claim 9, wherein the urging member includes a first spring which applies the force to the first cam and a second spring which applies the force to the second cam.
 16. The image forming apparatus according to claim 9, wherein the first roller and the second roller contact one the other with a delay time after the rear end of the sheet is pressed out from the first roller and the second roller, delay time being a time interval from the state that the cam surface of the second diameter of the first cam taking the second cam posture contacts the cam surface of the fourth diameter of the second cam taking the fourth cam posture to the state that the cam surface of the first diameter of the first cam taking the first cam posture contacts the cam surface of the third diameter of the second cam taking the third cam posture.
 17. A sheet conveying method which is carried out by a sheet conveyer comprising: a first roller which rotates in a first rotational direction; a second roller which contacts with the first roller to rotate in a second rotational direction opposite to the first rotational direction, the second roller and the first roller conveying a sheet by holding the sheet therebetween; a first cam configured to move with the first roller and to rotate against the first roller, the first cam having a cam surface of a first diameter when taking a first cam posture in a state that the sheet is not present between the first and second rollers, and having a cam surface of a second diameter greater than the first diameter when taking a second cam posture in a state that the sheet is present between the first and second rollers; a second cam configured to move with the first roller and to rotate against the second roller, the second cam having a cam surface of the third diameter when taking a third cam posture in a state that the sheet is not present between the first and second rollers, the cam surface of the fourth diameter of the second cam when taking a fourth cam posture in a state that the sheet is present between the first and second rollers, the cam surface of the third diameter of the second cam contacting the cam surface of the first diameter of the first cam when the first cam takes the first cam posture and the second cam takes the third cam posture, the cam surface of the fourth diameter of the second cam further contacting the cam surface of the second diameter if the first cam when the first cam takes the second cam posture and the second cam takes the fourth cam posture; and an urging member configured to apply a force to the first cam to rotate the first cam against the first roller from the first cam posture to the second cam posture and also apply a force to the second cam to rotate the second cam against the second roller from the third cam posture to the fourth cam posture, the method including, rotating the first cam against the first roller and the second cam against the second roller so that the cam surface of the second diameter of the first cam comes to contact the cam surface of the fourth diameter of the second cam when the sheet is held between the first roller and the second roller; conveying the sheet by holding the sheet between the first roller and the second roller; and providing a delay time by the first cam and the second cam from the state that the cam surface of the second diameter of the first cam contacts the cam surface of the fourth diameter of the second cam until the cam surface of the first diameter of the first cam comes to contact the cam surface of the third diameter of the first cam comes to contact the cam surface of the third diameter of the second cam when the rear end of the sheet is pushed out from the first roller and the second roller. 